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MG MGB GT V8 Factory Originals Technical - Servo Sticking - Factory V8

Hi all,

Having a small problem with the Car. The rear brakes have been sticking on, especially after a short stop in traffic, I try to accelerate and she bogs down. To get over the problem for now I've backed off on the rear wheel adjusters but I don't like the extra travel. So what should I do, is a rebuild something that can be done at home or is it a specialised job?

I went over a couple of the V8 register notes, and saw the problem mentioned, something to do with the earlier servo not being too reliable (valve problem), and the later B's had an improved dual valve version. Unfortunately I don't have the edition outlining how to rebuild the servo. Any suggestions?

74 V8
Neil Cotty

The servo is the only thing on my car that I haven't replaced in 16 years of ownership and I know very little about it.

Logic suggest that if there is a way in which the servo can jam the rear brakes, it would jam the front brakes at the same time.

If the adjusters were too tight (you ought to be able to spin the wheels after adjustment) then slackening off would restore normal braking. If the travel is too long you either have a lot of tired hoses (remember the hose in the middle at the back) or rusted cylinders.

Rusted cylinders or a broken shoe retainer would be my first area of investigation.

keep us posted

Roger Walker

I would agree with Roger on possible rust or crud in the rear cylinders, I would also add the possibilities of weak shoe return springs or a severely occluded brake hose. One of the problems with front calipers sticking on is due to the brake hoses filling up with crud. This "arteriosclerosis" prevents fluid pressure bleed off when the pedal is released. Your rear flex hose may have this problem also.

Roger is also right that on a single circuit brake system like the GT V-8s, a sticking servo would cause all the brakes to lock on. One of the biggest causes for servo failure is use of DOT 5 silicone fluid. AE Lockheed put out a position paper that specifically warns against using silicone fluid in any system equipped with a remote servo. They will also refuse to warranty a servo that has been used with silicone fluid. There may be a reprint of the text of this paper in the Archives.

I believe that Lockheed also found that the early servos had too much friction in the air valve plunger, which originally contained two O-ring seals and was prone to sticking. I think they eliminated one of the seals on later units.

Hope this helps,
Paul Kile
Paul Kile

Same as the other thread. As Paul suggested check rear brake hose.

The Servo serves front and rear brakes. master cylinder shares a common return spring internally.
Not to say that it could be sticking wheel cylinders (rare when both stick) But also check brake adjustment when to tight the shoes expand and etc... long shot but possible.
Bill Guzman


Thanks for the advice so far. I replaced both rear wheel cylinders to try and solve the fault as that was my first guess. I'm running brand new aeroquip s/s hoses on the rear and the two front. As for the fronts locking on with the rears when the servo malfunctions, I guess that's correct - I only presumed the rears were locking on. I backed the rears off earlier today and went for a short drive, she still had a funny little surge (a delay) after I came off the brakes but I (incorrectly) assumed this had solved the problem.

Now I've just came back from a longer drive and the problem is still there just not as bad, so I guess the fronts are also locking on. I think as I have new hoses and rear cylinders that the only cause left is either the servo or calipers.

FWIW, the servo occasional makes a small popping noise (air escaping) when applying the brakes, the pedal then drops suddenly, only a small amount mind you, maybe 1/2". Doesn't happen very often. I also notice when the brakes stick on, the pedal sometimes goes rock solid. Bizarre! Possibly the front caliper pistons sticking? I'm still with the Servo for now but I have had bad performance before with rusted caliper pistons.

BTW, the front Calipers on my V8 are Gold in colour and so are my front shocks.. Was that factory? I doubt it - ? My B and A are both Silver.

Looking forward to tomorrow, 30 MG's arriving from the UK on the MGOC run to Galway! I'll be there for it, with my digital cam. For anyone who wants to see, hassle me next Tuesday! <G>

74 V8
Neil Cotty

I had this problem years ago on an Austin 1800 (that shows how long ago it was)The set up was exactly the same as an MGB. The cause was that the plunger in the servo would jam every so often. I split the casing & found that the case or the plunger was out of round. By uncoupling the plunger I was able to twist it until I found a position where it didn't foul any more. After reconnecting everything it worked fine & was trouble free. In my case the servo wasn't so old ,so I didn't replace anything, only lube. In your case I'd be putting a recon kit in it. I believe the valve in the MC can also cause this to happen ,only in your case, it sounds more like the servo to me.HTHs Barrie E


Thanks for the info. It turned out to be the Servo. I had continued problems on the MGOC trip to Galway so Paul H. and I plugged the Vacumn line and the problem disappeared. I actually prefer the brakes unassisted as all my other MG's (apart from my CGT) are unassisted. I will just find a neater way to plug the line and leave the rebuild until I get the car back to Sydney in the next couple of years. Thanks for the help everyone. :)

74 V8
Neil Cotty

For some time the servo on my roadster has tended to stick like Neil's, and having experienced the torture of Dublin traffic on a warm day I can now definitely say mine is heat related. Neil will probably be horrified to hear I was driving close behind him yesterday using my handbrake instead of footbrake! Having driven Neil's car without servo (and shattered a few windows in Howth on the way) I am surprised how little difference it makes.

Paul Hunt


I was interested to read your comment re the Lockheed position on DOT 5 silicone fluid. I've owned my current 74 factory V8 for about a year and I've continued using silicone fluid during this time on the advice of the previous owner! Car is due for service soon and the pedal is pretty solid now. Any further advice, particularly whether anyone knows of a link to the Lockheed notes, would be gratefully received!

On a sunnier note, weather has been good enough here this week for my first chance this season of sustained driving with the webasto sunroof open - almost as fun as my old roadsters!!!!

Don Bryson

You might want to check the Archives in both the V-8 and MGB sections for "brake fluid" or "DOT 5". I know that someone reiterated the text of the AP Lockheed warning bulletin about using DOT 5 silicone fluid.

Paul Kile
Paul Kile

The information is contained in all AP Lockheed brake part catalogues. Don, check you mail box.

Roger Parker

This thread was discussed between 03/05/2001 and 10/05/2001

MG MGB GT V8 Factory Originals Technical index

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